1. Field of the Invention
This invention relates to an austenitic stainless steel useful for diesel and gasoline engine valves which provides the novel combination of excellent strength, hardness, resistance to oxidation and resistance to sulfidation at temperatures of 1100.degree.F and higher.
2. The Prior Art
Among the steels currently used for diesel and gasoline engine valves are those sold under the registered trademark ARMCO 21-4N, 21-2N, 21-12N, Silchrome 10, INCO 751, and N-155. A recent experimental alloy developed by Crucible Steel Co., designated as DV-2B, has undergone testing.
ARMCO 21-4N and 21-2N steels, and modifications thereof, are disclosed in U.S. Pat. Nos. 2,495,731; 2,603,738; 2,657,130; 2,671,726; Re. 24,431; 2,839,391 and 3,149,965. In broad ranges these alloys comprise 0.08 to 1.5% carbon, 3 to 20% manganese, 12 to 30% chromium, 2 to 35% nickel, up to 0.6% nitrogen, and balance substantially iron. Modifications include a low-silicon type, a molybdenum-containing type, a boron-containing type, a high-silicon type and a high-phosphorus type. The commercial 21-4N alloy has a nominal analysis of about 0.5% carbon, 9.0% manganese, 21% chromium, 3.75% nickel, 0.45% nitrogen, and balance substantially iron.
Silchrome 10 has a nominal analysis of about 0.4% carbon, 1% manganese, 20% chromium, 8% nickel, 3% silicon, residual nitrogen, and balance substantially iron.
INCO 751 has a nominal analysis of about 0.1% carbon, 1.0% manganese, 15.5% chromium, 72% nickel, residual nitrogen, 1% columbium, 2.3% titanium, 1.2% aluminum, and balance substantially iron.
N-155 has a nominal analysis of about 0.1% carbon, 1.5% manganese, 21chromium, 20% nickel, 0.15% nitrogen, 19.5% cobalt, 2.95% molybdenum, 1.15% columbium, 2.35% tungsten, and balance substantially iron.
Crucible DV-2B has a nominal analysis of about 0.5% carbon, 12% manganese, 21% chromium, 1% silicon, 0.45% nitrogen, 1% columbium, 2% tungsten, 0.4% vanadium, and balance substantially iron.
While several of these prior art alloys perform satisfactorily as diesel engine valves at temperatures below about 1300.degree.F, none has proven entirely satisfactory as a valve material in high performance diesel engines wherein exhaust valve temperatures range from about 1500.degree. to 1650.degree.F, and wherein valves are exposed to sulfide-containing fuels and lubricants. For example, ARMCO 21-4N, 21-2N and 21-12N have been found to lack adequate oxidation resistance and high temperature strength. INCO 751, while possessing good oxidation resistance and high temperature strength and hardness, has extremely poor sulfidation resistance. Silchrome 10 exhibits adequate oxidation resistance but relatively poor high temperature strength and sulfidation resistance. N-155 has good oxidation resistance, strength and hardness, and apparently has adequate sulfidation resistance, but is extremely high in cost. Crucible DV-2B has poor high temperature strength and oxidation and sulfidation resistance.
Recent changes in the compositions of fuels and lubricants have resulted in a demand for a steel of reasonable cost which exhibits good sulfidation resistance, together with the usual requirements of strength, hardness and oxidation resistance at temperatures above 1300.degree.F. From the above discussion of prior art alloys, it is apparent that such a steel is not now available.